Some home appliances, such as washing machines, use processes to harden the surface of components for increased wear and/or corrosion resistance. Salt bath nitriding is one example of a process by which a case hardened surface can be created on a component.
Whirlpool has used salt bath nitriding on the spin tube of the gear case on some washing machines to increase the corrosion resistance of the spin tube, which is exposed to wash liquid during operation of the washing machine. The spin tube, which is made of cold-rolled steel, is welded in place before nitriding because the prior art had established that nitrided parts cannot be successfully welded. Welding a nitrided component may lead to an unacceptable level of porosity in the weld. DE 3429994 teaches that after salt bath nitriding, surfaces are no longer weldable. The textbook, Laser Welding: a Practical Guide confirms that nitrided surfaces are generally unsuitable for fusion welding processes, since the weld will remove the surface hardness in the vicinity of the weld.